The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to an improved fast spin echo pulse sequence.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant y of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the xy plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 k-space lines, or "views" can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and a number of variations on this pulse sequence are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735. Unfortunately, even when state-of-the-art fast gradient systems are used, the EPI pulse sequence has difficulties with eddy current dependent and susceptibility induced image distortions.
A variant of the echo planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill ("CPMG") sequence, while EPI methods employ gradient recalled echoes. This fast spin echo pulse sequence ("FSE") is generally considered a problem free technique for acquiring multiple views with one excitation. It is, for instance, much less sensitive to field inhomogeneities and gradient timing errors than echo planar imaging. Further, because the readout gradient is always positive, unlike echo planar imaging, gradient fidelity is less of a problem.
However there are several critical parameters with the fast spin echo pulse sequence, which, if set incorrectly, can produce considerable image artifacts. These involve the radio frequency (RF) pulse spacing and phase relationships, and also the areas of the readout gradient pulses. Firstly, it is necessary that the time between the centers of the RF excitation pulse and first RF refocusing pulse should be half the time between the centers of adjacent refocusing pulses. Secondly, the RF phase angle between the RF excitation and RF refocusing pulses should be 90.degree.. Related to these requirements is the fact that the area of the readout gradient pulse between the excitation and the first RF refocusing pulse should be equal to half the area of the readout gradient pulse between each of the subsequent RF refocusing pulses.
For conventional fast spin echo imaging the above critical parameters can be controlled in a relatively straight forward manner. However, there are a number of imaging situations where the required degree of phase control between the RF pulses is difficult to achieve. Two such situations are (a) diffusion weighted imaging, where large gradient pulses are employed and resulting eddy currents are more prevalent, and (b) spectral-spatial excitation for fat suppression, where precise excitation pulse phase control is required for off iso-center imaging. The difficulty is usually caused by the presence of eddy currents which induce gradient fields. Norris, et al. proposed in "On the Application of Ultra-fast RARE Experiments," Magn. Reson. Med., 27, 142-164 (1992) a method for controlling the RF phase in an FSE pulse sequence which involved separating out two coherence NMR signal pathways, and using only one of the coherence NMR signals. One problem with this approach is the strong oscillation of the NMR signal amplitude which, if uncorrected, causes severe ghosting in the image. A similar idea has been proposed by Shick in "SPLICE: Sub-second Diffusion-Sensitive MR Imaging Using a Modified Fast Spin Echo Acquisition Mode," Magn. Reson. Med., 38, 638-644 (1997) in which the NMR echo signal amplitude is increased. Alsop discloses in "Phase Insensitive Preparation of Single-Shot RARE: Application to Diffusion Imaging in Humans," Magn. Reson. Med., 38, 527-533 (1997) a method for reducing the oscillations in the amplitude of these NMR echo signals. These methods employ crusher gradient pulses which suppress one of two NMR signal components that are normally produced in a CPMG pulse sequence. As taught by Alsop, the amplitude of the remaining component can be maintained relatively constant despite variations in phase caused by preparatory sequences such as diffusion weighting or spectral-spatial fat suppression. However, the suppression of one CPMG signal component in these prior methods reduces the amplitude of the acquired NMR signal by one-half.